Thank you for your post, and I apologize for the late reply.
When using Instant Web Publishing, make sure port 80 is open in your firewall. FileMaker, Inc. has registered port 591, so you may consider just having this port open and specifying it in the URL. For example, http://<IP Address>:591
FileMaker does not encrypt the data. However, with access privileges, you can specify the fields, layouts, scripts, etc. that you want to view so others cannot see sensitive data.